Austenitic steel



Sept. 15, 1964 Filed Oct. 31, 1962 F. A. MALAGARI, JR

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I w, g WHO ISWHO -3ONV.LSIS3B United States Patent Ofifice 3,148,979 fia'tented Sept. 15, 1964 3,148,979 AUSTENE'HC STEEL Frank A. Malagari, n, Frceport, Pa, assignor t0 Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania Filed Oct. 31, 1952, Ser. No. 234,439 11 Claims. (Cl. 75 -124) This invention relates to a new composition of matter and relates in particular to a new austenitic steel which exhibits high strength and oxidation resistance properties.

Austenitic steels of the aluminum-manganese variety have recently been developed which possess strength properties far in excess of those possessed by the conventional chromium-nickel stainless steels. Such compositions develop higher strength by age hardening than the commercially available austenitic age hardening grades, and annealed tensile properties are also much higher than for such known austenitic stainless steels. The aluminummanganese austenitic steels were originally intended to be inexpensive stainless steels in that the relatively inexpensive and non-critical elements aluminum and manganese were substituted for the conventional chromium and nickel. Although such compositions have relatively poor corrosion and oxidation resistance properties, these steels, the most common of which contains from about 7% to 16% aluminum, 20% to 40% manganese, 0.01% to 1.1% carbon and the balance iron, have proved to have mechanical properties that far exceed those of the conventional austenitic stainless steels at both room and ele vated temperatures. For example, certain of the aluminum-manganese steels have been found to have yield strengths of up to about 70,000 p.s.i. at room temperature and up to about 69,000 p.s.i. at 649 C. (1200 F.) as compared to about 35,000 p.s.i. and 12,000 p.s.i. respectively for AISI Type 304 stainless steel. Being austenitic, such aluminum-manganese steels work harden and, since they contain substantial quantities of aluminum, possess a low density. Hence, these steels not only possess superior strength properties to stainless steel, but also are lighter.

Although the oxidation resistance of the aluminummanganese steels at relatively elevated temperatures has been alleged to be good, such properties have been found to be inferior to stainless steel at temperatures exceeding about 871 C. (1600 F.). Such lack of oxidation resistance has markedly detracted from the advantageous properties of the aluminum-manganese steels since most applications of steel requiring high strength properties, particularly at elevated temperatures, also require good oxidation resistance.

It has now been found that the oxidation resistance of certain of the aluminum-manganese steels may be significantly increased by additions of critical amounts of both chromium and nickel. Hence, the present invention is directed to a low density, oxidation resistant, ductile austenitic steel having aluminum, manganese, chromium, nickel and carbon present within specified critical ranges.

The present compositions are not only ductile and formable, but have strength properties exceeding those of comparative stainless steels and oxidation resistance properties exceeding those of the known aluminum-manganese compositions and the austenitic stainless steels.

Additionally, it has been discovered that the steel of the present invention possesses electrical resistivity properties which, when coupled with its oxidation resistance, renders it an ideal electrical heating element material. Such steel has been found to possess significantly improved properties over the more expensive nickel-base alloys being employed for such applications. The present steel has also been found to possess a coefficient of resistance change vs. temperature property that is far superior to similar properties of the conventional heater materials. Still further, since the present steel possesses a substan tially austenitic structure, it sulfers no magnetostriction change in physical dimensions due to the influence of alternating current, and consequently is an ideal material for use as heating elements or other electrical ap plications where noise is a factor.

Such discoveries are significant since the aluminummanganese-chromium-nickel-carbon steel of the present invention is ductile and may be readily formed into wire, and the electrical resistivity is significantly higher than the Well known nickel-20% chromium alloy, referred to hereinafter as Alloy A, and which is now commonly employed for such applications as electrical heater units. The oxidation resistance properties of the composition of the present invention are equivalent to or exceed those of Alloy A. Further, the present composition is less expensive than Alloy A which contains about 80% nickel and 20% chromium, both of which are relatively expensive and critical elements.

It is, accordingly, an object of the present invention to provide a ductile, oxidation resistant, high strength, low density steel.

It is also an object of the present invention to provide a ductile and forrnable austenitic steel which possesses higher strength properties than stainless steel and excellent oxidation resistance properties.

A further object of the present invention is to provide a formable steel thatmay be fabricated into wire that has good electrical resistance and oxidation resistance properties that render it suitable for use as electrical resistance heating elements.

A still further object of the present invention is to provide a material having an excellent coefficient of resistance change vs. temperature properties.

Still another object of the present invention is to provide an austenitic steel having the property of being free from noise-emitting magnetostriction changes inphysical dimensions when subjected to alternating electric currents.

Other objects and advantageous features will be obvious from the following description and the accompanying drawings wherein:

FIGURE 1 is a graph showing the rate of oxidation of a steel of the present invention as compared with a known iron-carbon-manganese-aluminum steel at temperatures ranging from 760 C. (1400 F.) to 1149 C. (2100 F.);

FIG. 2 is a graph showing the ellect of varying the chromium contents of the steel of the present invention on oxidation resistance and illustrates the necessity for and criticality of the chromium range;

FIG. 3 is a graph showing the effect of varying the nickel contents of the steel of the present invention on oxidation resistance and illustrates the necessity for and criticality of the nickel range;

FIG. 4 is a graph showing the efifect of varying the aluminum content of the steel of the present invention on oxidation resistance and illustrates the necessity for and criticality of the aluminum range; 1

FIG. 5 is a graph showing the combined elfects of nickel, chromium and aluminum and the ellect of such elements within their critical range limitations, on the oxidation resistance of the present compositions;

FIG. 6 is a lower left hand portion of a triaxial diagram showing the oxidation resistance properties of steels containing four different aluminum contents and having varying nickel and chromium contents; and

FIG. 7 is a graph the curves of which represent the coefiicient of resistance vs. temperature for a steel of the present invention and for the Alloy A identified herein before.

In general this invention relates to a new steel having an analysis in the range of 0.50% to 1.0% carbon, 17 to 30% manganese, 8% to 12% aluminum, 1.0% to 12% nickel, 3% to 11% chromium and the balance iron with incidental impurities. Preferably, in order to obtain optimum oxidation resistance, the analysis is within the range of 0.75% to 1.0% carbon, 17% to 30% manganese, 9% to 12% aluminum, 3% to 8% nickel, to chromium, and the balance iron with not more than 0.5% of incidental impurities. In the steels of thisinvention carbon and manganese are required in the ranges given in order to stabilize the austenitic structure of the steel. Thus if the steel contains less than 0.5% carbon, the steel may contain undesirable quantities of unstable austenite and transformation products thereof. Preferably the carbon should be maintained at not less than 0.75% although not in excess of about 1.0% as greater amounts may have an adverse effect on workability and the corrosion resistance prop.- erties. Likewise, the manganese should be maintained at not less than about 17% so as to maintain a stable austenitic structure but should not exceed about 30% as greater amounts thereof will have an adverse effect on the strength properties of the resulting steel.

The steels of this invention are characterized by high oxidation resistance, good strength and ductility, low density, high electrical resistivity, low coefficient of resistance change vs. temperature, and are free of magnetostriction change in dimensions when subjected to alternating electrical currents.

As stated hereinbefore the present steel is an improvement over the known aluminum-manganese-carbon-iron steels. A representative steel of the prior known system is one (Heat No. 9X686) having an analysis of 0.92% carbon, 24.25% manganese, 0.013% phosphorus, 0.005% sulfur, 0.16% silicon, 9.8% aluminum, 0.20% chromium and the balance iron. In order to show the improvement of the present steel in oxidation resistance, reference is made to a steel (Heat No. GB-94) having an analysis of 1.02% carbon, 24.54% manganese, 0.014% phosphorus, 0.009% sulfur, 0.038% silicon, 9.72% aluminum, 3.56% nickel, 5.01% chromium and the balance iron. Such steels were subjected to different test temperatures in air for times of from 60 to 100 hours and the weight gain of the oxidation was measured. Referring to FIG. 1, the results of such oxidation tests are graphically illustrated by curves 10 and 12 representing the weight gain in trig/cm. on the steels at the different test temperatures with curve 10 being based on Heat No. 9x686 and curve 12 being based on Heat No. (BB-94. As clearly illustrated, the controlled addition of chromium and nickel greatly improves and imparts outstanding oxidation resistance to the steel.

' In order to more. clearly illustrate the efiect of chromium on the steel of this invention, reference may be had to the following Table I in which are listed a number of steels having chromium contents ranging from0% to 10% for a composition in which carbon is 0.78% to 1.03%, manganese is 16.9% to 27%, aluminum is 8.74% to 9.8% and nickel is 2.8% to 3.85% and the results of oxidation tests thereon at 1093 C. (2000 F.) for times in excess of 50 hours with the exception of FR-34 which was tested only 1 hour.

Table I Weight Heat C M11 P S 51 Al Ni Cr Gain, N0. hf g! FR34 97 23. 97 013 0. 150 9. 32 3. 85 10. 10 GJ40 1. 01 16. 93 020 009 070 9. 46 2. 92 4. 90 0. GJ-BS- l. 02 19. 58 030 009 100 9. 00 2. 88 5. 01 1. 03 (313-94.. 1. 02 24. 54 014 009 038 9. 72 3. 56 5. 01 0. 50 GI42. 78 25. 69 014 014 085 9. 73 3. 46 5. 13 0. i6 GI44 1. 03 26. 15 017 014 047 9. 80 3. 42 7. 25 1. 10 Gil-45... 1. 03 26. 18 018 017 100 9. 26 3:43 9. 34- 0. 91 RV-760 99 24. 00 006 009 045 8. 74 3. 49 9. 99 0. 21

Table III Weight Heat G Mn P S 51 Al N1 Or am, No. mg] 0111.

.010 .010 .030 4.01 7.02 5.08 183.90 .013 .000 .100 4.12 2.02 5.04 102.00 .010 010 120 4.20 3.01 5.00 175.50 .000 .012 000 4.17 3.20 0.04 110.00 .015 .011 033 4.17 7.88 10.03 05.50 .000 .012 000 7.10 2.01 5.04 110.15 000 .000 003 7.00 3.44 10. 03 113. 51 000 .000 .045 s. 74 3.40 0.00 .21 030 .000 .100 0.00 2.88 5.01 1.03 RV-649-.- .00 10.03 .000 .012 .070 0.00 2.07 5.04 0.0 (in-1s. .04 21.00 000 .014 .170 0.25 7.02 5.10 .10 GL-l7 .07 22.41 .011 .011 .050 0.25 7.00 10.31 1.11 GL-l 1.00 22.12 .010 .010 .052 0.02 11.68 5.00 1.05 (Id-40. 1.01 10. 00 .020 .000 .070 0.40 2.02 4.00 0.0 (31-42... 78 25.00 .014 .014 025 0.73 3.40 5.13 .10 613-04.-.. 1.02 24.54 .014 .000 .030 0.72 3.50 5.01 .00 RV762 .00 25.11 .007 .000 .038 11.40 3.50 5.18 .10

Referring to FIG. 2, curve 14 is a graphic illustration of the oxidation tests on the steels of Table I and clearly illustrates that the chromium content should be maintained in excess of 3% and preferably in the range of 5% to 10% for best results. Above 10% chromium appears to have a detrimental effect on the ductility of the resulting steel. 7

The effect of nickel is illustrated by reference to the steels of the following Table II and in which the nickel varies from 0.15% to 12% Where the other elements are in the range of 0.78% to 1.02% carbon, 8.6% to 9.75% aluminum, 16.93% to 27.36% manganese and from 4.9% to 5.36% chromium and in which the oxidation results as tested at 1093 C. (2000 F.) for times in excess of 50 hours are listed.

Table [I Nickel in controlled amounts is necessary along with the chromium to impart the desired oxidation resistance to the steel. Referring to FIG. 3, curve 16 is a graphic illustration of the oxidation test results on the steels having the nickel contents as listed in Table 11. As illustrated, the nickel content must be'not less than about 1% in order to maintain an oxidation rate of not more than 10 mg/cm. when subjected to a 1093 C. (2000 F.) temperature with the best results being obtained Where the nickel is maintained in the range of 3% to 8%.

Aluminum has proven to be critical in the steel of this invention as is illustrated by reference to the following Table III which lists steels having the aluminum varying from 4% to 12% with 0.75% to 1.02% carbon, 16.93% to 25.92% manganese, 2.0% to 11.7% nickel and 5% to 10% chromium and the oxidation rate for the steels as tested at ,1093" C. (2000 F.) for a time in excess of 50 hours.

Referring to FIG. 4, curve 18 is agraphic illustration of the oxidation test results reported in Table III as plotted against the aluminum content. Curve 18 demonstrates that aluminum is just as critical in the present steel as the chromium and nickel contents. Thus with only 4% aluminum, the oxidation rate is greater than mg./cm. .even With the chromium at 10% and is greater than rug/cm. where the chromium is at 5%. In order to maintain the oxidation rate at not more than 3) l mg./cm. as tested at 1093 C. (2000 F.), it is necessary to maintain the aluminum content at not less than 8% and, for best results, preferably in the range of 9% to 12% with the nickel and chromium within the limits specified.

. such rate are classified as bad with the symbols representing different levels of aluminum as indicated.

Referring to the plots on FIG. 6, it becomes clear that the required oxidation rate is not obtained simply by increasing nickel and chromium but that instead the alumi- While the curves of FIGS. 2, 3 and 4 clearly demonnum content is also critical. Thus as indicated GL-l6 strate the effect of each of chromium, nickel and alumiwhich has about 10% chromium and about 7.9% nickel num, respectively, on the oxidation resistance of the steel but only about 4% aluminum has a poor oxidation rate of this invention, it has been found that there is a definite of 95.5 mg./cm. whereas a comparable steel GL-17 relationship of the total of these elements within the 10 having about 10% chromium and about 7.9%.nickel but ranges stated and particularly the preferred ranges to in which the aluminum is 9.25% has an excellent oxidainsure the optimum oxidation resistance. Thus for contion rate of 1.11 mg./cm. The same comparison may sistent and optimum oxidation resistance the elements be made for the plots of GL-10 and GL-18, the main difmust be present within the ranges given in an amount so ference therebetween being that GL-lO having only about that the sum of Ni+Cr-|-(4 Al) must be equal to or 4% alurninurn has a poor oxidation rate and GL-18 havgreater than 43 in order to insure that the oxidation rate ing about 9.25% aluminum with the other elements being is not in excess of 10 mg./cm. as tested at 1093 C. approximately the same as those for GL-lt), has an excel (2000 F.) for from 50 to 100 hours. lent oxidation rate. Thisis also pictorially illustrated in In order to demonstrate the necessity for the foregoing a comparison of RV761 and RV-76O as well as by the relationship, reference may be had to the following Table 20 comparison of GL8 and GL-9 with RV-698 and a comlV in which certain of the steels of the foregoing tables parison of RV'648 with Gl-38 and 61-40. FIG. 6 as well as additional steels are listed in an order dependclearly illustrates the necessity to maintain the elements ent upon an increase in the value of Ni+Cr+(4 Al). within the critical limits previously described.

Table IV Weight Heat N0. 0 Mn P 3 31 Al Ni Or 5111, [Ni+Or+(4 Al)] rug/cm.

The foregoing relationship is demonstrated graphically While it has been stated that the nickel may be present in FIG. 5 in which the measured oxidation rates of the in an amount ranging from 1% to 12%, 3% is preferred steels of Table IV are plotted against the sum of 50 as the lower limit in order to obtain fairly consistent Ni+Cr+(4 Al) and a parameter is drawn to show the oxidation resistance properties. On the other hand, while definite band resulting. As is clearly evident from FIG. up to 12% nickel will impart excellent oxidation resist- 5 only those steels having the elements within the ranges ance properties, it is preferred to limit the nickel to not given hereinbefore and in which Ni+Cr+(4 Al)43 f more than 8% since such element is relatively expensive will have the required low oxidation rate. as compared to iron. Further since nickel is an austen- While reference has been made hereinbefore to the itizer and aluminum and chromium are ferritizers in ironeffect of nickel and chromium, it must be emphasized that base steel alloys, it has been found that where more than the oxidation resistance of the steel cannot be improved about 8% nickel is utilized, that the nickel enters into a simpiy by increasing the quantity of either or both of second magnetic phase along with the aluminum and chrothese elements. Instead the aluminum content is also 60 mium with the result that embrittlement and cracking critical and essential regardless of any such increase in is sometimes encountered during the processing of the the nicrcel and chromium contents. While this has been resulting steel which contains in excess of 8% nickel. substantiated by the showing made in FIGS. 2 and 5, Since the aluminum and chromium, where present in the it becomes clearer when the results obtained on steels upper portion of the ranges given, cooperate to eliect a containing four different levels of aluminum are examined. duplex structure of austenite as well as the second mag For this purpose the steels identified hereinbefore in the netic phase with the nickel Where the nickel is over 8%, specification and in Tables I, II, III and IV have been that the combined chromium and aluminum contents plotted in FIG. 6 which, while shown as a parallelogram should not exceed about 19.5% so as to prevent the is actually the lower left hand portion of a triaxial diaformation of abrittle composition. gram in which chromium and nickel contents of the diifer- 7 The steel of this invention may be annealed at temperaent steels are plotted against the balance of their compositures of 1038 'C. (1900 'F.) to 1149 C. (2100 F.) tions and for purpose of clarity the different heat numbers and air cooled or water quenched therefrom to soften it are identified on the parallelogram for a ready comparito place it in a condition for forming into diiferent shapes son. Any steel having an oxidation rate not in excess of or for cold drawing into wire, the time of the anneal 10 mg./cm. is classified as good and those in excess of being dependent upon the mass to be so formed or drawn.

for heat radiation.

For example, a rod of the steel of Heat GB94 when annealed forv 30 minutes at 1093" C. (2000 F.) and water quenched had a R hardness of 29, a 0.2% yield strength of 99,400 pounds per square inch, a tensile Strength of 133,700 pounds per square inch, a percent elongation in 1 inch of 51 and a reduction of area of 59%.

The steel of this invention has a high room temperature electrical resistivity of from 130 to 160 microhm-centimeters; For example, a typical steel as represented by the composition of Heat GB-94 has a room temperature resistivity of 156.8 microhm-centimeters as compared to the room temperature resistivity of 100 microhm-centimeters of the well known Alloy A referred to hereinbefore. Since the steel of this invention has a high resistivity, the steel is useful as the heating element in space heaters of all types including furnaces, ovens and stoves or heaters for home or other building heaters. In contrast to the well known Alloy A, since the steel of this invention has such a high resistivity a given heater element formed from a wire or ribbon of my steel can have a diameter or cross sectional area 50% larger than a similar heater element formed from Alloy A with a resulting increase in strength and surface area available Such an increase in cross sectional and surface area makes it possible to operate the resulting heater in heat exchanger or power control applications at considerably lower temperatures than similar heaters formed of Alloy A for a given length of heater element and power or B.t.u. output. This characteristic found with the steel of the present invention is of extreme importance where maximum current levels and maximum temperatures must be considered in the design of the heaters.

It has also been found that the steel of this invention has a smaller coefficient of resistance change vs. temperature than that of conventional heater materials. For example, a representative steel of this invention Heat No. GK20 having a density of 6.724 gm./cm. and having an analysis of 0.97% carbon, 22.98% manganese, 0.020% phosphorus, 0.011% sulfur, 0.13% silicon, 9.24% aluminum, 3.26% nickel, 5.18% chromium and the balance iron when formed into a heater wire and tested had an electrical resistivity of 144.4 microhm-centimeters and a coefficient of resistance change over a temperature range of 80 C. (112 F.) to +100 C. (+212 F.) as shown by curve 20 of FIG. 7. Such resistance change may be readily compared to that of the well known Alloy A as illustrated by curve 22 of FIG. 7. As will be seen there is substantially zero change in the coeflicient of resistance of the steel of Heat No. GK-20 over the temperature range of -19 C. (2.2 F.) to +20 C. (+68 F.) and that above such temperature any change therein is at a substantially constant and considerably lower rate than that of Alloy A. Likewise, it is seen that the steel of Heat No. GK-20 has a total change in coeificient of resistance of only about 0.13 ohm/ohm as measured over the wide range of 40 C. (-40 F.) to +45 C. (+113 F.). Such steel, having a low coefiicient of resistance change up to 100 C. (212 F.), is readily formed into precise wire and tape resistors of all types and is useful as meter shunts as well as in high wattage, self-supporting or other cast grid, wire or tape resistors.

As is evident from curve 20 of FIG. 7, the steel represented by Heat GK-20 has a negative coefiicient of re sistance at temperatures below 0 C. (32 P.) which renders it particularly suitable for use in temperature compensating and temperature sensing devices. In addition since the steel has an austenitic structure it has been found that it is free from magnetostriction dimensional changes when subjected to alternating electric currents thereby rendering it suitable for applications where magnetostriction noise is objectionable.

As will be appreciated and as shown in the analysis of the steels given in the foregoing tables the steel of this invention will usually contain small amounts of incidental impurities. Thus the alloys may contain up to 0.04% phosphorus, up to 0.03% sulfur and up to 0.50% silicon. Preferably phosphorus is maintained at less than 0.02% maximum, silicon at less than 0.20% maximum and sul' fur at less than 0.015% maximum. However, in some instances where it is desired to improve machinability, the sulfur may be up to 0.5%. Usually phosphorus, sulfur and silicon will be present in a total amount of not more than 0.5%. In the composition ranges and analysis given hereinbefore, the percentages given in all instances are weight percentages.

I claim:

1. A low density, ductile, high strength, austenitic steel consisting essentially of, 0.50% to 1% carbon, 17% to 30% manganese, 8% to 12% aluminum, 1% to 12% nickel, 3% to 11% chromium, and the balance iron with incidental impurities, the combined chromium and aluminum contents of said steel being less than about 19.5% when the nickel content is more than about 8%, said steel being characterized by high oxidation resistance at temperatures in excess of 871 C. (1600 F.), high electrical resistivity, low coefiicient of electrical resistance change vs. temperature and being substantially free from mag netostriction change in dimension when subjected to alternating electrical currents.

2. A low density, ductile, high strength, austenitic steel consisting essentially of 0.75% to 1% carbon, 17% to 30% manganese, 9% to 12% aluminum, 3% to 8% nickel, 5% to 10% chromium, and the balance iron with incidental impurities, said steel being characterized by high oxidation resistance at temperatures in excess of 871 C. (1600 F.), high electrical resistivity, low coeflicient of electrical resistance change vs. temperature and being substantially free from magnetostriction change in dimension when subjected to alternating electrical currents.

3. A low density, ductile, high strength, austenitic steel consisting of, 0.50% to 1% carbon, 17% to 30% manganese, 8% to 12% aluminum, 1% to 12% nickel, and 3% to 11% chromium, 0.04% maximum phosphorus, 0.03% maximum sulfur, 0.2% maximum silicon, and the balance iron, the combined chromium and aluminum contents of said steel being less than about 19.5% when the nickel content is more than about 8%, said steelbeing characterized by high oxidation resistance at temperatures in excess of 871 C. (1600 F.), high electrical resistivity, low coefficient of electrical resistance change vs. temperature and being substantially free from magnetostriction change in dimension when subjected to alternating electrical currents.

4. A low density, ductile, high strength, austenitic steel consisting of, 0.75% to 1% carbon, 17% to 30% manganese, 9% to 12% aluminum, 3% to 8% nickel, 5% to 10% chromium, 0.02% maximum phosphorus, 0.015% maximum sulfur, 0.2% maximum silicon, and the balance iron, said steel being characterized by high oxidation resistance at temperatures in excess of 871 C. (1600 F.), high electrical resistivity, low coefficient of electrical resistance change vs. temperature and being substantially free from magnetostriction change in dimension when subjected to alternating electrical currents.

5. A low density, ductile, high strength, austenitic steel consisting essentially of, 0.50% to 1% carbon, 17% to 30% manganese, 8% to 12% aluminum, 1% to 12% nickel, 3% to 11% chromium, and the balance iron with incidental impurities, the combined chromium and aluminum contents of said steel being less than about 19.5% when the nickel content is more than about 8% and the sum of the nickel and chromium contents plus four times the aluminum content is not less than 43, said steel being characterized by high oxidation resistance at temperatures in excess of 871 C. (1600 F.), high electrical resistivity, 10W coefficient of electrical resistance change vs.

9 temperature and being substantially free from magnetostriction change in dimension when subjected to alternating electrical currents.

6. A low density, ductile, high strength, austenitic steel consisting essentially of, 0.75% to 1% carbon, 17% to 30% manganese, 9% to 12% aluminum, 3% to 8% nickel, 5% to chromium, and the balance iron with incidental impurities, the sum of the percentages of the chromium content and nickel content plus four times the aluminum content being not less than 43, said steel being characterized by high oxidation resistance at temperatures in excess of 871 C. (1600 F.), high electrical resistivity, low coefficient of electrical resistance change vs. temperature and being substantially free from magnetost'riction change in dimension when subjected to alternating electrical currents.

7. A heating element formed of an austenitic steel having an electrical resistivity of from 130 to 160 microhmcentimeters and high oxidation resistance, said steel having a composition analysis consisting essentially of 0.50% to 1% carbon, 17% to 30% manganese, 8% to 12% aluminum, 1% to 12% nickel, 3% to 11% chromium, and the balance iron with incidental impurities, the combined chromium and aluminum contents of said Steel being less than about 19.5% when the nickel content is more than about 8%.

8. A heating element formed of an austenitic steel having an electrical resistivity of from 130 to 160 microhmcentimeters and high oxidation resistance, said steel hav- 10 ing an analysis consisting essentially of from 0.75% to 1% carbon, 17% to manganese, 9% to 12% aluminum, 3% to 8% nickel, 5% to 10% chromium and the balance iron with incidental impurities.

9. A low density, ductile, high strength, austenitic steel consisting of, about 1.0% carbon, about 24.5% mang nese, about 9.70% aluminum, 3.5% nickel, 5.0% chromium, and the balance iron with incidental impurities.

10. A low density, ductile, high strength, austenitic steel consisting of, 0.50% to 1% carbon, 17% to 30% manganese, 8% to 12% aluminum, 1% to 12% nickel, 3% to 11% chromium, up to 0.5% sulfur, and the balance iron with incidental impurities, and in which the sum of the nickel and chromium contents plus four times the aluminum content is not less than 43.

11. An electrical resistor heating element formed of an austenitic steel having a composition consisting essentially of, 0.50% to 1% carbon, 17% to 30% manganese, 8% to 12% aluminum, 1% to 12% nickel, 3% to 11% chromium, and the balance iron with not more than 0.5% incidentalimpurities, and in which the sum of the nickel and chromium contents plus four times the aluminum content is not less than 43, and in which the combined chromium and aluminum contents are less than about 19.5 when the nickel content is more than about 8%, said heating element being characterized by having an electrical resistivity of from to microhmcentimeters.

No references cited. 

1. A LOW DENSITY, DUCTILE, HIGH STRENGTH, AUSTENITIC STEEL CONSISTING ESSENTIALLY OF, 0.50% TO 1% CARBON, 17% TO 30% MANGANESE, 8% TO 12% ALUMINUM, 1% TO 12% NICKEL, 3% TO 11% CHROMIUM, AND THE BALANCE IRON WITH INCIDENTAL IMPURITIES, THE COMBINED CHROMIUM AND ALUMINUM CONTENTS OF SAID STEEL BEING LESS THAN ABOUT 19.5% WHEN THE NICKEL CONTENT IS MORE THAN ABOUT 8%, SAID STEEL BEING CHARACTERIZED BY HIGH OXIDATION RESISTANCE AT TEMPERATURES IN EXCESS OF 871*C. (1600*F.), HIGH ELECTRICAL RESISTIVITY, LOW COEFFICIENT OF ELECTRICAL RESISTANCE CHANGE VS. TEMPERATURE AND BEING SUBSTANTIALLY FREE FROM MAGNETOSTRICTION CHANGE IN DIMENSION WHEN SUBJECTED TO ALTERNATING ELECTRICAL CURRENTS. 